The present invention relates to a hydraulically operated, rotary hammer (percussion) drill, particularly for drilling anchor holes (e.g. roof holes, lewis holes, block holes), the drill including a striking piston and a control unit which alternatingly switches the direction of movement of the striking piston. The insertion end of a tool is charged by the striking piston and displaceable in the longitudinal direction of the striking piston as well as driven by means of a rotation mechanism.
A significant field of use for hammer drills is the so-called drilling of anchor holes in which large drill diameters (under certain circumstances including a double rod assembly composed of nested bore hole tubing) and great drilling depths are involved.
A problem that arises in this connection is that if high power consumption occurs at both working units, i.e. the striking mechanism and the rotation mechanism, the transfer elements of the rotation mechanism (drilling tool; insertion end for drilling tool) may become overloaded, even though the hammer drill is generally powerful enough and suitable for such stresses. Moreover, there exists the danger, when drilling with high rotary power, that additionally occurring resistances, due to changes in the material being drilled (for example, working through soft, hard or tough layers) or due to jamming, cause the drill rods to seize up. Such an occurrence results in a considerable amount of time spent to free the drilling rods and possibly in damage to them or their complete loss.
Thus, it is necessary, for the field of use in question, to reduce the energy of each individual stroke of the hammer drill without excessively reducing the number of strokes if the power of the rotation mechanism or its torque rises above a predetermined limit. Too great a drop in the number of strokes generated by the striking mechanism would adversely influence the vibratory behavior of the drilling rods, such vibratory behavior being necessary to overcome the friction between the outer cylinder of the rods and the material being drilled.
Hydraulically operated drilling and striking devices are known from German Pat. No. 2,658,455 in which the energy of each individual stroke can be changed by an externally supplied control pressure while simultaneously changing the number of strokes. The reduction of the energy of each individual stroke is here effected by a step-wise actuated shortening of the stroke of the striking piston connected with an increase in the number of strokes. Used for hammer drills, this type of control has the drawback that the increased number of strokes results in greater wear and friction welding at the already mentioned transfer elements caused by high areal pressures in conjunction with a large number of minute movements in the axial direction. Additionally, the step-wise switching to shorten the working stroke has an adverse effect on the drilling process.